This invention relates to airplane door latch mechanisms, and particularly to an automatic clutch for a latch handle that allows the door to be opened by an exterior latch handle without back-driving the interior latch handle, but also allows the interior latch handle to control the motion of the latch linkage in both directions during operation of the door using the interior latch handle.
Airplane doors for pressurized passenger jet liners are designed to resist air pressure greater on the inside of the door than the outside. For this reason, plug-type airplane doors have been developed which open inwardly so that the inside air pressure tends to hold the door closed, rather than relying on a hinge or latch mechanism on an outwardly opening door to resist the inside air pressure.
Some inwardly opening doors are mounted to translate into the cabin and into the space above the ceiling panels when opening to afford unimpeded access to the exit. An example of such a door is shown in U.S. Pat. No. 4,470,566 issued to James T. Fitzgerald and entitled "Plug-Type Aircraft Door Actuating Mechanism." This door is hung from a trolley which follows tracks to guide the door into the overhead space when the door is unlatched. A counterbalance spring acting through a cable drum facilitates manual opening and closing of the door.
The door shown in the Fitzgerald patent has an interior latch handle for opening the door from the inside of the airplane, and an exterior latch handle for use by airline service personnel for opening the door from the outside. The exterior latch handle is normally disengaged from the door latch linkage, so that operation of the interior latch handle does not cause the exterior latch handle to move. The reverse is not true, however. The interior latch handle should always be capable of opening the airplane door without any special procedures that could confuse passengers and delay opening of the door, so it should never be decoupled from the latch linkage. Therefore, operation of the exterior latch handle will backdrive the latch linkage, and the interior latch handle will be backdriven to rotate upwardly into the passenger space in the cabin. A passenger who happened to be sitting in the seat adjacent to the interior latch handle, when the exterior handle was operated, could be struck by that backdriven interior handle. This invention eliminates that possibility by decoupling the latch linkage from the exterior latch handle in the opening direction of the latch linkage when the handle is in its "home" position, so the latch linkage cannot drive the handle when it is in its home position, and by providing a one-way drive coupling between the latch linkage and the interior latch handle in the opening direction of the handle so the handle can always drive the latch linkage from the closed position thereof.
The latch linkage, through which the interior and exterior latch handles operate the door latch, includes an overcenter mechanism whose purpose is to ensure that any opening forces acting on the door will not open the door, but instead will be reacted against a fixed stop. As the interior latch handle is rotated by hand in the opening direction, the overcenter crank is rotated away from the fixed stop and the opening forces exerted on the door will be exerted on the interior latch handle in opposition to the opening rotation. When the over-center mechanism passes dead-center (at about 55 degrees of handle rotation) the over-center mechanism reverses the direction of the door-opening forces in the latch linkage, so that those forces tend to rotate the latch handle in the opening direction. If the handle is not restrained after passing the dead-center position of the overcenter crank mechanism, the latch linkage and the latch itself will shift suddenly to the fully unlatched position, resulting in mechanical impact and noise in the latch mechanism. The noise could alarm the passengers and could accelerate the wear on the linkage and associated mechanisms. This invention prevents the opening forces from being suddenly released when the overcenter mechanism passes dead-center, by automatically converting the latch handle from a one-way drive, which it is at the beginning of its motion, to a two-way drive before the dead-center position on the overcenter mechanism is reached, so the handle can restrain the latch linkage and control the rate of unlatching.
The operation of the latch handle must be simple and obvious, even to untrained passengers. Moreover, the operation of the door latch handle should give the passenger or crew member tactile and visual clues that the handle is being operated properly. That is, it must operate smoothly when swung in the correct direction, but strongly resist movement in any other direction. It must also present a moderate resistance to movement to prevent accidental or idle operation, or operation by young children playing with the handle. Finally, it should give a tactile feel through the handle that it is actually operating something, rather than just the zero resistance feel of a dummy handle, to give the operator a reassuring feed-back that he or she is operating the handle correctly. Each of these desirable features, present in the existing handle shown in the Fitzgerald patent, is retained in the automatically clutched handle of this invention.